[ns] "Mac/802_11e cfb_ to 1" is wrong in some scenes ?

[=?GB2312?B?zsK+sMjd?=]

 Hi, now I am working on 802.11e in NS2. But I think i have find a bug about
it.

When I set Mac/802_11e cfb_ to 1 (in tcl/lan/ns-mac.tcl)  , I set a random
scene, sometimes  it will tell me that
 " *assertion " dh->dh_fc.fc_type == MAC_Type_Control" failed : file
"mac/802_11e/mac-802_11e.cc",line 80*
*    Aborted (core dumped)*  "

But in some other scenes it will well .  I have try some scenes , just find
it seems to happen in the same scene when the interval is enough

small,like 0.05,which means that 20 packets should be sended per second,
while donot happen when the interval is large like 0.2 .

  I want to know if I am right,and I want to find a method to solve this
problem as soon as possible.
  It is kind of you for your reply.

  Best regards,

Wen